Protestors hold signs during a vigil to support the closing of the Vermont Yankee nuclear power plant at the Statehouse Monday, Jan. 23, 2012 in Montpelier / TOBY TALBOT, The Associated Press

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MONTPELIER — The Vermont Yankee nuclear plant once again is refusing the state's request that it conduct more tests for radioactive tritium in a former drinking water well on the plant grounds.

Christopher Wamser, site vice president for plant owner Entergy Corp., says in a Jan. 20 letter to Public Service Commissioner Elizabeth Miller that such testing would be inappropriate because it could contaminate the bedrock aquifer at the bottom of the well and might not produce reliable results.

"We do not see how the incremental value of the results from such testing would outweigh the risk, particularly when there is already extensive testing information available from numerous other wells on- and off-site (including drinking water wells)," Wamser wrote in a letter released by Miller's department.

A key issue in the dispute is what type of testing would be most appropriate.

The plant says it would want to use a method that requires purging the well, which could increase the chances of the bedrock aquifer becoming contaminated. Wamser said the state's suggestion that a "grab sample" be taken from the well wouldn't produce reliable results.

Wamser wrote that "vertical flow within the well and insertion of the sampling equipment would cause mixing within the well column and would not tell us the location of tritium, if any, within that column."

Laurence Becker, the state geologist, said in an interview that Wamser's reading of the situation is wrong. He said he had checked with the federal Environmental Protection Agency and been told that the amounts of tritium in the well water would be consistent.

"The EPA got back to us about sampling for tritium in a water column. It's mixed very completely," Becker said. "You can take a grab sample from what is sitting in the bore hole right now and get a sense of what the tritium levels are."

Becker said the state is especially interested in new results for the well, which is deeper than other test wells dug around the plant since tritium was first discovered to have leaked in late 2009.

Entergy officials had produced a conceptual site characterization showing that water flows up from the bedrock into shallower underground depths where tritium has been found. The theory was that with water flowing upward, tritium — essentially a radioactive form of water — would not be flowing downward to the bedrock.

That's why testing of a well dug into bedrock is important, Becker said, especially when one earlier test showed tritium in it.

"Maybe it's an anomaly, maybe it's not," he said.

Gov. Peter Shumlin first requested follow-up tests of the former drinking water well in late 2010, while he was still governor-elect. Several requests by the state since then also have been rebuffed.

Wamser's Jan. 20 letter came one day after a federal judge in Brattleboro issued a ruling saying Vermont may not force its lone nuclear plant to shut down when its initial 40-year license expires March 21.